The structure and mechanism of several multicenter oxidation reduction systems are being investigated by spectroscopic and kinetic techniques. The main emphasis is on the enzymes beef heart cytochrome oxidase, yeast complex II and milk xanthine oxidase. The experimental approach emphasizes electron paramagnetic resonance, magnetic circular dichroism, Resonance Raman and optical spectroscopy combined with potentiometric and stoichiometric oxidoreductive titrations and rapid reactions. Current effort is directed towards (1) Characterisation of the binulcar center of cytochrome oxidase; (2) The definition of an EPR signal probably arising from the hitherto EPR undetectable copper; (3) Spectroscopic characterization of the several intermediate detected during the reoxidation of reduced cytochrome oxidase using MCD and Raman spectroscopy; (4) A study of the allosteric relationships between cytochrome a, cytochrome a3 and Cuu; (5) The relationship of the potentiometric properties of solubilized cytochromes cl and b to th hemes of Complex III; (6) An analysis of the kinetics of reoxidation of xanthine oxidase relating the kinetics of production of hydrogen peroxide and superoxide to the currently accepted mechanism for this enzyme.